Using advanced modulation methods, like discrete multitone and carrierless amplitude and phase techniques, asymmetrical-digital-subscriber-line (ADSL) service can move digital data at up to 8 Mbits/s continuously over conventional telephone lines, while allowing voice communications. Because ADSL is packet-switched rather than circuit-switched, it's also immune to congestion at the service provider. This opens the path to applications like real-time videoconferencing, Web hosting, video streaming, distance learning, telemedicine, and video-on-demand.
From a hardware perspective, ADSL is a digital service that's superimposed and thus "piggybacks" on the existing public switched telephone network. The modem's job is to handle signals moving upstream and downstream between the service provider and the subscriber's interface, which is typically a PC. Another key component in an ADSL connection is a splitter, which sits between the modem and the telephone line and separates the broadband digital signal from the analog voice signal (Fig. A).
In practice, an ADSL connection includes two ADSL modems, each called an ADSL Terminal Unit (ATU). One modem is at the central office (ATU-C), and the other is at the customer premises (ATU-R). Each ADSL modem contains a total of four main functional blocks (Fig. B). Of these blocks, the line interface, typically a transformer-based hybrid circuit, connects to the splitter. Next is the internal circuit, which generally includes an ASIC chip set and memory. The remaining blocks are the power supply and a data-line interface, typically PCI, USB, or Ethernet.
